Jack-up drilling platform

ABSTRACT

A jack-up type of marine drilling platform has a buoyant hull for use in transporting the platform to the drilling site, with longitudinally separable legs which project upwardly from the hull during transport. The drilling derrick or other upstanding structure is movable on the surface of the hull to adjust the center of gravity of the platform to accommodate environmental conditions. Also, such derrick or structure is used to lift and transport separated upper leg sections to and from stowage positions on the hull.

Umted States Patent 1191 [111 3,830,071 LeTourneau Aug. 20, 1974 [54] JACK-UP DRILLING PLATFORM 3,699,688 10/1972 Estes 6l/46.5

[75] Inventor: Richard L. LeTourneau, Longview, I

Primary Examiner.lacob Shapiro Attorne A em, or Firm-Bertram H. Mann; Frank B. [73] Assignee: IHC Holland-Letourneau Marine p g Corporation, Kilgore, Tex.

[22] Filed: Oct. 13, 1972 [57] ABSTRACT [21] Appl. No.: 297,225 A jack-up type of marine drilling platform has a buoyant hull for use in transporting the platform to the drilling site, with longitudinally separable legs which (SI/4656225114133 project upwardly from the hull during transport The [58] d 61/46 5 46 drilling derrick or other upstanding structure is movle 0 care able on the surface of the hull to adjust the center of gravity of the platform to accommodate environmen- [56] References and tal conditions. Also, such derrick or structure is used UNITED STATES PATENTS to lift and transport separated upper leg sections to ahorgon and from stowage-positions on the hull. 00 S011 3,628,336 12/1971 19 Claims, 17 Drawing Figures Moore 61/465 PATENTEDauszo I874 sum 02 av 10 PATENTED M1820 I974 sum 03 or 10 PATENTED M1320 I974 saw on or 10 sum .usor 10 N\ mm 7 QT) S Pmmmw am PATENTEBmzo mm sum 080F10 PATENTED M1820 1974 sum as 0f10 PATENTEDAUGZO I974 saw 109, or 10 BACKGROUND OF THE INVENTION This invention relates to mobile drilling platforms of the jack-up type having a floatable hull, legs which can be propelled from transport positions projecting above the hull downwardly to engage the water bottom to serve as bases for use in jacking up the bull to a stable drilling position substantially above the water surface. Generally, such platforms must be transported with the legs projecting high above the hull in quite unstable positions. There have been suggestions of separately transporting the legs or spuds on the hull, or upon separate barges and thereafter utilizing a crane to erect the legs. However, the use of a crane for this purpose has the substantial disadvantage that a leg or leg section, while suspended by the crane, is subject to swinging in a dangerous manner under the influence of wave and wind action on the hull and cable suspended leg section. Furthermore, the load line of a crane must be at tached to the top of the leg section to be detached so that the length of such section is limited to the feasible height of the crane.

SUMMARY OF THE INVENTION Accordingly, an object of this invention is to provide novel means for separating drilling platform legs into longitudinal sections.

Another object is to provide novel means for attachment to a separable leg section and to detach the section from the remainder of the leg.

Another object is to provide novel means for stowing a detached leg section upon the hull without releasing the leg section, even temporarily for swinging movement, as when a crane is used for this purpose.

Another object is to mount a derrick or other upstanding structure on a hull in such manner as to be movable to adjust the center of gravity of the platform to compensate for environmental conditions, as wave and wind action, or to weight a leg to test the water bottom.

In accordance with the present invention, the combination hull and platform has a plurality of legs (at least three) which may be drawn upwardly during transport of the platform to the drilling site, then propelled downwardly to the water bottom, at the site, and thereafter utilized forjacking up the hull substantially above the water level for use as a stable drilling platform. The drilling derrick or other upstanding structure, preferably, is centrally stored during water transport and is mounted on rails so as to be propelled to its drilling position, as over a slot at the edge of the hull-platform, or to the vicinity of one of the legs. A grapple device is mounted on the derrick or other structure and provided with means for securely gripping a separable upper section of the leg preferably intermediately, whereupon the derrick or other structure is again propelled to transport the detached leg section to a storage position on the hull. One or more sections of each leg may be detached and stowed in this fashion so that the legs do not project substantially beneath the hull during transport, nor do they project excessively above the hull in a manner to unstabilize the floating platform. The procedure is reversed at the drilling site to reassemble the legs and propel them to the marine bottom. The derrick or other structure, also, may be moved to the vicinity ofa leg for testing the strength ofthe water bottom.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings which illustrate the invention,

FIG. 1 is a perspective view of a mobile drilling platform according to the invention in transport condition with upper sections of each leg detached and stowed on the hull.

FIG. 2 is a plan view showing the top of the hull, with several positions of the derrick shown.

FIG. 3 is an enlarged elevation showing parts of the hull-platform and one of the legs or spuds in partially depressed position.

FIG. 4 is a schematic, enlarged perspective illustrating the leg chord drive arrangement.

FIGS. 5 and 6 are enlarged longitudinal sections on line 55 of FIG. 3 and showing also a device for insertion and removal of the joint pins.

FIG. 7 is an enlarged detail section taken on line 77 of FIG. 2.

FIG. 8 is a further enlarged detail section taken on line 88 of FIG. 2 showing the means for locking the movable longitudinal rail sections to the underlying transverse rails.

FIG. 9 is a cross-sectional detail taken on line 9-9 of FIG. 2.

FIG. 10 is an enlarged elevation showing a part of the drilling derrick with a leg section secured thereto and lifted from its underlying hull deck anchorage.

FIG. 11 is a plan view of the structure in FIG. 10.

FIG. 12 is a side view showing the carriage or crawler structure at the base of the derrick.

FIG. 13 is an enlarged section taken on line 13-13 of FIG. 14.

FIG. 14 is a half plan view of the structure in FIG. 12 taken on line l4-l4 thereof.

FIG. 15 is a sectional view taken on line 15-l5 of FIG. 14.

FIG. 16 is an enlarged plan view of one of the grapple arrangements taken on line 16-16 of FIG. 10, with the jaws shown in open position.

FIG. 17 is an elevation of the structure in FIG. 16 viewed from the line 17-17 thereof.

DETAILED DESCRIPTION OF THE DRAWINGS The hull-platform A, shown floating in FIG. 1, is of generally triangular horizontal shape and transversely box sectioned to provide buoyancy and space for crew, machinery, and supplies and has a slot B in its rear side through which the bore hole is drilled. At each corner of the hull, there is provided a spud or leg well, as at C in FIG. 3, in which are mounted for vertical reciprocation legs or spuds D, E, and F. Suitable verticallyspaced, opposed, right angle guides in the wells, one being shown at 18 in FIG. 4, slidably engage each chord of each leg. At the center of the hull there is provided the drilling derrick G movably mounted on a carriage or crawler base, generally designated H. The derrick consists of a special rectangular lower part and a tapered upper part. The legs D, E, and F are of generally triangular cross section. Also provided on the top deck of the hull are various operational features such as pipe racks 20 and a heliport 21. The legs have detached upper sections D E., and F anchored to the hull deck, as will be explained. Mounted above each spud .31 in FIG. 4, on one of the structural vertical, rectangular chords of the corresponding leg or spud, as at 32 in FIG. 4. At the lower end of each spud there is an en larged foot 33 which, in the uppermost position of the leg, is partially concealed within a generally conical recess 34 in the bottom of the hull so as to reduce as far as practicable the resistance to movement of the hull through the water during transport to the drilling site.

Also mounted on the top deck of the hull (FIG. 2) are parallel transverse rails 36, extending continuously between the spud wells C at the rear end of the hull (FIG. 3) and just rearwardly of drilling slot B. Intersecting these transverse rails are pairs of longitudinal rail sections 37, 38, and 39 forming continuous trackage extending from the spud well at the forward end of the hull to the rear end of the hull at the sides of drilling slot B. Longitudinal pairs of rail sections 37 and 39 are fixed on the hull deck, while the intermediate pair of longitudinal rail sections 38 are separately formed and rest upon transverse rail sections 36, as best shown in FIG. 7. Rail sections 38 may be slid laterally along transverse rails 36 in either direction and are provided with guide and stabilizing cleats 40 hooked around and underneath the edges of the transverse rails, and central reinforcements 41. Certain outer cleats 44 are provided with locking bolts 45. All of the transverse and longitudinal rails are provided with innerside toothed racks, as at 42 and 43 (FIG. 7), for a purpose to be described.

FIG. 10 is an elevation of the lower portion of drilling derrick G and its crawler base H shown slid with its tracks 38 to the vicinity of an upper leg section, as D to which it is attached. The lower, main body of the, derrick consists of rectangularly arranged vertical structural chords or columns 46, 47, 48, and 49 (FIG. 11) connected by variously arranged struts and braces, as at 50, 51, and 52. The vertical chords 46 49, at their bottoms, rest upon rectangularly arranged beams 53 (FIG. 12) mounting the drilling floor 54 on top thereof. Beams 53, in turn, rotatably rest upon a ring 55 having an annular rack 56 with which mesh drive pinions 57 driven by motors 58 secured to extensions 59 on beams 53 (FIG. 10). Ring 55 (FIGS. 12 and 14), in turn, slidably rests upon tracks 60 and 61 on the upper surface of derrick crawler-base H.

As shown in FIGS. 12 and 13, ring 55 has peripheral cleats, as at 62, embracing the edges of tracks 60. The ring, and with it the derrick, may be driven linearly along tracks 60, 61 by means of a pair of hydraulic motors 63 (FIGS. 14, 15), each secured at one end to rib or gusset 64 on the ring and with its piston rod 65 secured to a clevis 66. A horizontal web 67 rigid with the lower edge of the clevis has edge cleats68 encompassing edge flanges 69 on the corresponding crawler track 60. Brackets 70 depending from cleats 68 slidably abut the side walls of tracks 60. Brackets 70 and track walls 71 (FIG. 13) have perforations 72 and 73 which may be aligned to receive pins 74. Pins 74 are secured in place by pivoted retainer pieces 75. Brackets may be selectively secured by pins 74 in any of the holes 73a 73e (FIG. 15) in order to utilize the doubleacting hydraulic motors 63 to shift the ring and derrick in either direction on the crawler.

The two legs 47 and 48 at one side of the derrick lower portion are provided with lower reinforced portions, as 47a, 48a in FIGS. 10 and 11, which mount outturned racks 76 and 77. Mounted to travel vertically along leg or chord portions 47a and 48a is a climber device, generally designated K (FIGS. 10 and 11, 16 and 17). The body of the climber is made up of framing pieces as at 78, 79, and 80, and top and bottom members, as 82, 83, and 84, forming a box frame. The top and bottom framing pieces 82, 83, and 84 form cleats, as at. 85 and 86 in FIG. 16, embracing laterally extended guide flanges on outer plates 87 and 88 of derrick leg portions 47a, 48a, to guide and restrain the climber in its movements along the derrick. At the lower portion of each side frame member 80, there is provided a reinforced housing 91 mounting a pair of hydraulic motors 92 and gearing 93 which drive pinions 94 for propelling the climber device along derrick racks 76 and 77.

At the outer ends of each of the upper transverse framing members 82 and 83 (FIGS. 10, I1, 16, and 17), there are provided a pair of grapple jaws 95 and one such jaw is provided at the ends of lower frame members 84. Each jaw 95 has a short outer arm 96 and a longer inner arm 97. On the inner edge of shorter arm 96 there is provided a plate 98 with reinforcing ribs 98b and terminating in a short rack section 98a of two rack teeth facing inwardly at 45 to the corresponding upper framing member 82 or 83. A total of six such grapple jaws and two rack sections are provided. The rack sections, which I term rack mates, are disposed and positioned to mesh with the lifting racks on the chords of separable leg sections being handled, as will be explained. At the end of each longer inner arm 97 of the grapple jaws there is pivotally connected an angled grapple arm 99 having a drilled clevis at its free end in which is seated a pin 100 pivotally mounting the eye 101 of a bolt 102. This bolt is positioned to pass through a hole 103 in plate 98 and held therein by washer 104 and a nut 104a to secure the climber device to the leg chords with the chord rack meshing with rack mate 98a to prevent dropping of the climber device and its load. The six grapple devices are positioned to securely lock the leg section to the climber device against swinging movements. In order to draw the grapple jaws and arm 99 firmly about the encompassed leg chord the end of bolt 102 may be fastened to the piston rod 105 of an hydraulic motor 105a of the type shown in greater detail in FIGS. 5 and 6.

The spud legs D, E, and F are each divided into two or more longitudinal sections so as to be separated, as will be explained, to lower the center of gravity of the platform during transport in floating condition, and to be reassembled at the drill site, for jacking up the platform to drilling position. Each of the leg sections has mating joint elements at the upper and lower extremities of its chords (FIG. 10), except the top and bottom sections, which have one each. The bottom clevis elements 106 have pin apertures 106a and top tongue elements 107 have pin holes 108. Mounted on the top deck of the hull are joint lug elements 107a corresponding to top leg joint elements 107 on the leg sections. These deck lugs are arranged in groups of threes adjacent the respective spud wells, as indicated at L, M, N, O, P, and Q (FIG. 2). The six lug groups shown provide .for secure stowing in upright positions of two separable sections'of each leg. The deck lugs are firmly anchored to heavy vertical columns (not shown) extending deeply into the hull and rigidly secured thereto.

FIG. 3 shows an upper leg section as D joined to the bottom section D, with the mating joint elements tightly secured together by means of massive tapered pins 109 in collets 109a (FIGS. 5, 6). The assembly and disassembly of these huge joints is expedited by means of the bracket or cradle and press equipment illustrated. A bracket or cradle 110 is suitably secured, as by bolting, pinning, or welding, to the outer clevis joint element 106. A rack 111 is slidable through the rear wall 112 of the bracket and is engaged by a pinion 113 actuated by a ratcheted hand lever 114. The tapered pin, encompassed by collet 109a held in place by retainer rings 115, is laid in cradle 110 with its end inserted in the pin hole 1060 in the clevis element 106 and the outer end of the collet engaged by rack 111. The pin assembly is initially inserted by manipulation of ratcheted hand lever 114. The final setting of the pin is achieved by means of a double acting hydraulic motor 117 removably secured to side walls 116 of the cradle and with its piston rod 118 pinned as at 119 to apertured lugs 120 projecting from the joint pin. The assembly is maintained by means ofa nut 121 threaded on bolt 122 extending axially through the pin and securing the cover 123 in position. To withdraw the pin, these steps are reversed. The cradle may remain in place during stowage of the leg section and only when the section is firmly secured to the underlying section, preparatory to propelling of the leg through the leg well.

All of the spud legs are oftriangular cross section and each of the vertical corner chords is provided with a drive rack, as at 31, 31a, and 31b in FIG. 11, these being continued along the separable upper sections as at 31c, 31d, and 3112 in FIG. 11. It is especially important that the sections be secured together without any play and with their respective chord racks in accurately timed relationship for proper action of the leg drive mechanisms. The particular attachment means shown is well-calculated to achieve this function.

OPERATION FIG. 1 shows the hull-platform A floating, as in transport to a drilling site, with its spud-legs drawn upwardly as far as possible. The upper sections of the legs are removed and installed upright upon adjacent mounts on the hull deck. Only one separable section for each leg is shown for clarity, but anchorage is provided for two separable sections for each leg. Where the legs are of extreme length, they may be divided into even more longitudinal sections which are stowed upright on additional mounts so as to occupy a minimum of deck space. The drilling derrick is located centrally of the hull in its position of maximum stability. However, the derrick may be moved as desired to correct the trim of the platform or to compensate for wave and wind action. Upon reaching the drilling site, the hull is properly anchored with slot B over the section of sea floor to be drilled. The drilling derrick is then propelled rearwardly along longitudinal tracks 39, 38, and 37 to a po sition over slot B and between dismounted leg sections at positions L and M. The derrick is then rotated until climber-grapple device K faces the leg section to be remounted, say D, at position M. The climber is elevated sufficiently to grip dismounted section D preferably at about the longitudinal center thereof. The derrick and ring are then slid laterally along tracks 60 on the top of the crawler base H until rack mates 98a mesh with leg chord racks 31c and 31a. Finally, grapple arms 99 are swung around leg chords 32c and 32e, as in FIGS. 11 and 16, and secured by means of bolts 102 and nuts 104a. This firm gripping of the leg chords by the grapple may be enforced by hydraulic motor means, as at 105a (FIG. 16). The anchorages at the bottom of the dismounted section are then disconnected, again using the power devices of FIGS. 5 and 6, if desirable, and the leg section lifted by continuing the upward movement of climber device K a short distance. Thereafter, the derrick is slid forwardly with its load onto separate longitudinal track sections 38. These track sections with the derrick and load are then slid along transverse tracks 36 to the vicinity of bottom leg section D which has been partially lowered in its well. Finally, the leg 7 section carried by the derrick is mounted upon and securely fastened to underlying leg section D, recourse being had to the linear and rotary adjustments of the derrick to bring the complementary joint elements a, 107 together. When the joint pins are inserted with the aid of the powered cradles, the assembled leg elements are ready to be further depressed.

Similar procedures are followed in mounting dismounted leg sections upon their underlying leg sections of the other two legs so that these also can be lowered and additional leg sections for all legs may be provided, if desired. When all leg sections are standing on the bottom, the water bottom may be tested by successively moving the derrick into the vicinity of the respective legs. The hull is then elevated to its drilling position above the water level. Normally, drilling will be achieved with the derrick over slot B. However, under severe environmental conditions, it may be desirable to discontinue drilling and move the derrick to alter the center of gravity or wind reaction of the platform.

To prepare the platform for refloating, it is first lowered. Thereafter, the detachable leg sections are removed in order, with use of the derrick, and stowed upright on the deck, as in FIG. 1. Ultimately, the lowermost leg sections are elevated as far as possible, as in FIG. 1, whereupon the hull-platform is ready for transport.

Important advantages of the novel platform leg sectionalizing means result from the rigid securement of the movable derrick to the leg section to be detached or separated intermediately thereof, rather than at the top, as would be the case where a crane is used for the purpose. Thus, a very much longer detached section may be handled so that fewer detached sections need be stored on the deck. Of course, the movability of the derrick makes possible correction of trim problems, while reducing water ballasting, and is available to counteract hostile environmental conditions. Also, the separate sections may be maneuvered vertically as well as horizontally in joining the same to leg sections already in place, or removing such sections, so that it is not necessary to depress or elevate the entire barge for this purpose. Moreover, this joint construction may pass through the drive structures 1, J, etc., without interference with the leg guides, as at 18 in FIG. 4. The

novel leg joints are more fully described and claimed in a copending applicationassigned to the assignee of the present application, Ser. No. 363,987 filed May 25, 1973.

Various features as illustrated may be modified as will occur to those skilled in the art. For instance, the legs may be mounted e xteriorly of the hull and separation of the legs into multiple leg sections and re-uniting, as well as stowage on the deck, may be achieved by cutting and welding procedures rather than special attachments. Moreover, a structure other than the drilling derrick may be utilized in the leg dismantling, stowing, and re-uniting steps. Also the dismounted leg sections may be variously placed, for instance, immediately adjacent the leg wells, particularly where only one detachable leg section is provided. Finally, certain features of the invention may be utilized with leg drives other than the rack and pinion drives shown herein. The exclusive use of all modifications as come within the scope of the appended claims is contemplated.

I claim: I

1. In a mobile drilling platform, a hull, a plurality of legs movably secured to said hull, each of said legs having a plurality of separable longitudinal sections, means for shifting said legs longitudinally thereof relative to said hull for causing said legs to project varying distances above and below said hull, an upstanding structure, means for selectively-propelling said upstanding structure into the vicinity of said legs, a carriage movable along said upstanding structure, and grapple means on said carriage for securement to the upper sections of said legs for lifting and lowering the attached leg sections.

2. A mobile drilling platform as described in claim 1 in which said upstanding structure is a drilling derrick.

3. A mobile drilling structure as described in claim 1 in which said upstanding structure is mounted for rotation about its vertical axis to position said grapple means for securement to a leg section.

4. A mobile drilling platform as described in claim 1 in which said grapple means includes a plurality of legsecuring devices spaced substantially apart vertically for preventing swinging of a detached and supported leg section. I

5. A mobile drilling platform as described in claim 1 in which said legs are of truss construction each with a plurality of structural chords, said grapple means including horizontally spaced leg securing devices positioned to grip a plurality of said chords.

6. A mobile drilling platform as described in claim 1 in which said legs are of truss construction each with a pair of structural chords substantially in a plane, said grapple means including a plurality of leg securing devices positioned to grip each of said chords at areas substantially spaced apart longitudinally.

7. A mobile drilling platform as described in claim 1 further including complementary shoulder means on said upstanding structure and said upper leg sections positioned to interengage when said upstanding structure and a leg section are secured together, said grapple means further including a securing element for encompassing at least a part of said upper leg section and to maintain said shoulder means in engagement for resisting dropping of said leg section.

8. A mobile drilling platform as described in claim 7 in which said complementary shoulder means comprise interfittable rack elements.

9. A mobile drilling platform as described in claim 8 in which the rack elements on said upper leg section constitute parts of said means for shifting the said leg axially with respect to said hull.

10. A mobile drilling platform as described in claim 7 in which said securing element comprises a pivoted lever.

11. A mobile drilling platform as described in claim 1 further including detachably interlocking joint elements on abutting upper and underlying leg sections.

12. A mobile drilling platform as described in claim 11 in which joint elements corresponding to the joint elements at the top of the underlying leg sections are secured to said hull for anchoring detached upper leg sections in upstanding positions.

13. A mobile drilling platform as described in claim 5 in which said legs are of triangular section each with three corner chords connected by bracing.

14. A mobile drilling platform having a floatable hull and a plurality of legs movably mounted on said hull, powered means for shifting said legs longitudinally relative to said hull to cause said legs to variably protrude above and beneath said hull and to lift said hull when said legs are'on bottom, a drilling derrick movably mounted on said hull, said legs each having multiple longitudinal sections detachably secured together, grapple means movably mounted on one side of said derrick including elements for encompassing at least a part of an upper leg section, second powered means for moving said derrick to and from its drilling position and positions adjacent said legs, and third powered means for propelling said grapple means up and down on said derrick, first for bringing said grapple means abreast an upper section of a leg, then lifting said upper section to separate it from its underlying section after said grapple means is secured to said upper section, and then, after said derrick is moved away from said underlying section, lowering said upper section onto said hull for lowering the center of gravity of the platform.

15. A mobile drilling platform as described in claim 14 in which said legs extend through wells in said hull, and further including powered drive pinions and racks mounted respectively on each well wall and the corresponding leg, and said grapple means including a rack section for meshing with the rack on the leg section to which it is secured to resist dropping of said secured leg section.

16. A mobile drilling platform as described in claim 14 in which said derrick is of truss construction, including at least a pair of structural chords on one side, said grapple means comprising a carriage riding along said latter chords and guide means engaging longitudinally spaced parts of said latter chords to resist separation of said carriage from said derrick.

17. A mobile drilling platform as described in claim 14 further including a base for said derrick slidable along said hull, said derrick being movably mounted on said base for improved juxtaposition between said derrick and the platform leg being manipulated.

18. A mobile drilling platform as described in claim 14 further including tracks on said hull extending from a central position on said hull to a drilling recess spaced therefrom and also to positions adjacent said legs, and

bearing elements on the bottom portions of said derrick from the underlying section and lift the leg upper section clear.

moving the derrick so as to move the secured upper leg section away from the underlying section,

lowering the secured section on to the hull,

fastening the secured section to the hull in upstanding position,

and releasing the fastened section from the derrick,

then reversing the steps when on location to successively lower the legs, utilizing the derrick to add leg sections as needed, and lifting the hull when the legs are on bottom. 

1. In a mobile drilling platform, a hull, a plurality of legs movably secured to said hull, each of said legs having a plurality of separable longitudinal sections, means for shifting said legs longitudinally thereof relative to said hull for causing said legs to project varying distances above and below said hull, an upstanding structure, means for selectively propelling said upstanding structure into the vicinity of said legs, a carriage movable along said upstanding structure, and grapple means on said carriage for securement to the upper sections of said legs for lifting and lowering the attached leg sections.
 2. A mobile drilling platform as described in claim 1 in which said upstanding structure is a drilling derrick.
 3. A mobile drilling structure as described in claim 1 in which said upstanding structure is mounted for rotation about its vertical axis to position said grapple means for securement to a leg section.
 4. A mobile drilling platform as described iN claim 1 in which said grapple means includes a plurality of leg-securing devices spaced substantially apart vertically for preventing swinging of a detached and supported leg section.
 5. A mobile drilling platform as described in claim 1 in which said legs are of truss construction each with a plurality of structural chords, said grapple means including horizontally spaced leg-securing devices positioned to grip a plurality of said chords.
 6. A mobile drilling platform as described in claim 1 in which said legs are of truss construction each with a pair of structural chords substantially in a plane, said grapple means including a plurality of leg securing devices positioned to grip each of said chords at areas substantially spaced apart longitudinally.
 7. A mobile drilling platform as described in claim 1 further including complementary shoulder means on said upstanding structure and said upper leg sections positioned to interengage when said upstanding structure and a leg section are secured together, said grapple means further including a securing element for encompassing at least a part of said upper leg section and to maintain said shoulder means in engagement for resisting dropping of said leg section.
 8. A mobile drilling platform as described in claim 7 in which said complementary shoulder means comprise interfittable rack elements.
 9. A mobile drilling platform as described in claim 8 in which the rack elements on said upper leg section constitute parts of said means for shifting the said leg axially with respect to said hull.
 10. A mobile drilling platform as described in claim 7 in which said securing element comprises a pivoted lever.
 11. A mobile drilling platform as described in claim 1 further including detachably interlocking joint elements on abutting upper and underlying leg sections.
 12. A mobile drilling platform as described in claim 11 in which joint elements corresponding to the joint elements at the top of the underlying leg sections are secured to said hull for anchoring detached upper leg sections in upstanding positions.
 13. A mobile drilling platform as described in claim 5 in which said legs are of triangular section each with three corner chords connected by bracing.
 14. A mobile drilling platform having a floatable hull and a plurality of legs movably mounted on said hull, powered means for shifting said legs longitudinally relative to said hull to cause said legs to variably protrude above and beneath said hull and to lift said hull when said legs are on bottom, a drilling derrick movably mounted on said hull, said legs each having multiple longitudinal sections detachably secured together, grapple means movably mounted on one side of said derrick including elements for encompassing at least a part of an upper leg section, second powered means for moving said derrick to and from its drilling position and positions adjacent said legs, and third powered means for propelling said grapple means up and down on said derrick, first for bringing said grapple means abreast an upper section of a leg, then lifting said upper section to separate it from its underlying section after said grapple means is secured to said upper section, and then, after said derrick is moved away from said underlying section, lowering said upper section onto said hull for lowering the center of gravity of the platform.
 15. A mobile drilling platform as described in claim 14 in which said legs extend through wells in said hull, and further including powered drive pinions and racks mounted respectively on each well wall and the corresponding leg, and said grapple means including a rack section for meshing with the rack on the leg section to which it is secured to resist dropping of said secured leg section.
 16. A mobile drilling platform as described in claim 14 in which said derrick is of truss construction, including at least a pair of structural chords on one side, said grapple means comprising a carriage riding along said latter chords and guidE means engaging longitudinally spaced parts of said latter chords to resist separation of said carriage from said derrick.
 17. A mobile drilling platform as described in claim 14 further including a base for said derrick slidable along said hull, said derrick being movably mounted on said base for improved juxtaposition between said derrick and the platform leg being manipulated.
 18. A mobile drilling platform as described in claim 14 further including tracks on said hull extending from a central position on said hull to a drilling recess spaced therefrom and also to positions adjacent said legs, and bearing elements on the bottom portions of said derrick for riding on said tracks.
 19. The method of operating a mobile drilling platform, having a movable drilling derrick and legs with separable lower and upper parts, selectively, for transportation and drilling comprising the steps of shifting the legs upwardly relative to the hull, moving the derrick adjacent a leg, securing the derrick intermediately to the leg upper section, utilizing the derrick to detach the leg upper section from the underlying section and lift the leg upper section clear, moving the derrick so as to move the secured upper leg section away from the underlying section, lowering the secured section on to the hull, fastening the secured section to the hull in upstanding position, and releasing the fastened section from the derrick, then reversing the steps when on location to successively lower the legs, utilizing the derrick to add leg sections as needed, and lifting the hull when the legs are on bottom. 